1. Field of the Invention
The present invention relates to a lens apparatus for moving one or more moving lenses of a lens system up to the end position of its moving course by a servo-motor.
2. Description of the Prior Art
A zoom lens for TV camera comprises at least one moving lens element such as a variator and compensator movably mounted within a cam tube. In zoom lenses hitherto known, these movable lens elements are conventionally supported by a slide member such as ball bearings through the cam slots of a translation cam tube and rotation cam tube. The rotation cam tube is driven by a servo-motor through a gearing mechanism. A speed instruction signal of a predetermined level is applied to the servo-motor through a servo-amplifier so as to rotate the rotation cam tube by an amount corresponding to the signal. Thereby, the moving lenses are displaced axially to perform a zoom function.
When the moving zoom elements are shifted from a first position to a second position, according to the above described conventional method, they are moved at first at a constant speed to a predetermined position from the first position and then they are moved to the second position from the predetermined position at a converging speed gradually decreasing to zero (0) speed. The predetermined position at which the phase of lens movement is changed over from the constant speed to the diverging speed is fixed and remains unchanged for all possible moving speeds at which the moving zoom elements are moved. This involves some problems. The higher the moving speed is, the larger the mechanical inertia of a moving part is. Therefore, large mechanical vibration and noise are produced when the zoom elements being moved at a high speed is stopped suddenly at the predetermined position. Many attempts have been made to smoothly drive and stop the moving lenses in a zoom unit. One solution to this problem hitherto known and most widely employed at present is to extend the end part of the cam slot formed on the rotation cam tube in the circumferential direction of the cam tube so that the moving zoom elements may be stopped effectively even when the rotation cam tube still continues rotating. However, this solution has a disadvantage with respect to the rise time of lens movement. Since the cam slot has an extension extending in the circumferential direction, the start of lens movement is delayed and there is brought about a reverse effect. Due to the presence of the circumferential extension of the cam slot, the lenses can not be driven at once when the servo-motor starts rotating and there is produced a delay in response. This is an important drawback involved in the solution.